1. Field of the Invention
The present invention relates to a valve structure for use in a liquid feedstock vaporizer mainly used in fabricating semiconductor devices and the like.
2. Description of the Prior Art
In the fabrication of semiconductor devices, CVD apparatus (chemical vapor deposition apparatus) play a very important role and are often used as one of film-forming apparatus. Liquid compounds such as TEOS, TEB and TMOP are used as film-forming materials. In using such liquid compounds, they are required to be vaporized by any means.
A liquid vaporizer/feeder incorporating a combination of a mass flowmeter and a vaporizer has been developed and widely used as one of such vaporization means. A typical system for feeding gas into a CVD apparatus in a semiconductor device fabricating process is schematically illustrated in FIG. 1. Although the vaporizer in FIG. 1 is constructed in accordance with the present invention which will be descried later, other components are similarly to conventional ones and, hence, the conventional system will be explained with reference to this drawing for convenience.
A liquid feedstock 106 such as TEOS in a liquid container 105 is pressurized by a pressurizing gas 107 (typically helium ) and fed into a liquid mass flowmeter 101. A flow rate control signal from the liquid mass flowmeter 101 is transmitted to a vaporizer 100 in which the flow rate of the liquid feedstock 106 is controlled to a given value. A carrier gas 108 comprising an inert gas is fed through a carrier gas mass flowmeter 104 into the vaporizer 100.
When an open/close valve incorporated in the vaporizer 100 is made open, the feedstock vaporized therein, together with the carrier gas, is fed into a CVD apparatus 102, and a film-forming process is initiated. Since the film-forming process is performed under a reduced pressure, a reaction chamber is always evacuated by a vacuum pump 108. The pressure in the CVD apparatus can always be checked by means of a pressure gauge 103.
FIG. 9 shows the main portion of a conventional vaporizer, in which an area filled with a liquid is referred to as a liquid area A, an area in which the liquid is vaporized is referred to as a vaporization area B, and an area through which the gas generated by vaporizing the liquid is carried by a carrier gas is referred to as a carriage area C.
Liquid feedstock 106 delivered from the liquid mass flowmeter 101 is introduced through a liquid inlet passage 20 into the vaporizer 100. There is provided a flow rate control valve body 23 for controlling the flow rate of the liquid feedstock 106 flowing from the liquid area A into the vaporization area B. A spring 29 acts to depress the flow rate control valve body 23 to enlarge the clearance between a valve seat 27 and a seat-abutment portion 23b of the flow rate control valve body 23 so as to provide communication between the liquid area A and the vaporization area B.
On the other hand, a control plunger 25 is kept in contact with an end portion 23a of the flow rate control valve body 23 via a diaphragm 26 by means of a flow rate control valve driving element. Accordingly, when the control plunger 25 is moved upward in the drawing, the flow rate control valve body 23 is pressed upwardly against the biasing force of the spring 29, so that the clearance between the liquid area A and the vaporization area B is closed.
Thus, the flow rate of the liquid feedstock 106 into the vaporization area B is accurately controlled by controlling the operation of the flow rate control valve body 23 through the control plunger 25.
The liquid feedstock 106 thus flowing into the vaporization area B is vaporized, carried by the carrier gas 108 through the carriage area C, and discharged through the outlet passage 22 into the CVD apparatus 102.
In the carriage area C is provided a shut-off valve body 24 for completely shutting off the liquid feedstock 106. The shut-off valve body 24, when depressed by an open/close plunger 24b connected to an open/close control driving element, shuts off the communication between the vaporization area B and carriage area C thereby to completely stop the film-forming process.
The feeding of the liquid feedstock 106 is thus controlled by using both the flow rate control valve having the flow rate control valve body 23 and the shut-off valve having the shut-off valve body 24 because the flow rate of the liquid feedstock 106 must be controlled accurately to a minute level by the flow rate control valve and, hence, the shut-off valve is provided dedicatedly for the shut-off function.
However, the provision of the shut-off valve within the carriage area C causes a disadvantage. That is, since the shut-off valve body 24 is located right above the vaporization area B in a manner to cover the area B, the evacuation efficiency in the vaporization area B is likely to decrease and hence, the internal pressure in the vaporization area B is prone to increase.
Generally, the vaporization efficiency becomes higher with a lowering pressure. Accordingly, the increase in the pressure within the vaporization area B hinders the vaporization of the liquid feedstock. As a result, the liquid feedstock will partially remain as it is without being vaporized. Such residual liquid 28 bumps in the vaporization area B and further impinges on the inner wall of the carriage area C or of the shut-off valve body 24, resulting in instantaneous vaporization. This causes a pressure fluctuation to occur in which the pressure increases and then immediately decreases.
The pressure fluctuation reaches the CVD apparatus 102 and observed at a pressure gauge 103 provided therein. Such a pressure fluctuation will seriously affect the film-formation; that is, a large pressure fluctuation results in a non-uniform semiconductor surface or an uneven film thickness, thus leading to a decrease in the yield of wafers.
It is, therefore, an object of the present invention to provide a vaporizer having a high vaporization efficiency and a less pressure fluctuation.